1. Field of the Invention
This invention relates to a method and the product produced by said method. More specifically, this invention is directed toward the preparation of a photoconductive composite having three distinct layers.
2. Description of the Prior Art
The formation and development of images on the imaging surfaces of photoconductive layers by electrostatic means is well known. The best known of the commercial processes, more commonly known as xerography, involves forming a latent electrostatic image on the imaging surface of an imaging member by first uniformly electrostatically charging the surface of the imaging layer in the dark and then exposing this electrostatically charged surface to a light and shadow image. The light-struck areas of the imaging layer are thus rendered relatively conductive and the electrostatic charge selectively dissipated in these irradiated areas. After the photoconductor is exposed, the latent electrostatic image on this image-bearing surface is rendered visible by development with a finely divided colored marking material, known in the art as "toner". This toner will be principally attracted to those areas on the image-bearing surface having a polarity of charge opposite to the polarity of charge on the toner particles.
The developed image can then be read or permanently affixed to the photoconductor where the image layer is not to be reused. This latter practice is usually followed with respect to the binder type photoconductive films (e.g. zinc oxide/insulating binder resins) where the photoconductive imaging layer is also an integral part of the finish copy, (U.S. Pat. Nos. 3,121,006 and 3,121,007).
In so called "plain paper" copying systems, the latent image can be developed on the imaging surface of a reusable photoconductor or transferred to another surface such as a sheet of paper and thereafter developed. When the latent image is developed on the imaging surface of a reusable photoconductor, it is subsequently transferred to another substrate and then permanently affixed thereto. Any one of a variety of well known techniques can be used to permanently affix the toner image to the copy sheet including overcoating with transparent films and solvent or thermal fusion of the toner particles to the supportive substrate.
In the above "plain paper" copying systems the photoconductive material used in the photoconductive insulating layer should preferably be capable of rapid switching from insulating to conductive to insulating state in order to permit the cyclic use of imaging surface. The failure of a material to return to its relatively insulating state prior to the succeeding charging/imaging sequence will result in an increase in the rate of dark decay of the photoconductor. This phenomenon, commonly referred to in the art as "fatigue", has in the past been avoided by the selection of photoconductive materials possessing rapid switching capacity. Typical of materials suitable for use in such a rapidly cycling imaging system include anthracene, sulfur, selenium and mixtures thereof (U.S. Pat. No. 2,297,691), selenium being preferred because of its superior photosensitivity.
In addition to anthracene, other organic photoconductive materials, most notably, poly(N-vinylcarbazole), have been the focus of increasing interest in electrophotography, U.S. Pat. No. 3,037,861. Until recently, neither of these organic materials have received serious consideration as an alternative to such inorganic photoconductors as selenium, due to fabrication difficulties and/or to the relative lack of speed and photosensitivity within the visible band of the electromagnetic spectrum. The recent discovery that high loadings of 2,4,7-trinitro-9-fluorenone in poly(vinylcarbazoles) dramatically improves the photoresponsiveness of these polymers has led to a resurgence in interest in organic photoconductive materials, (U.S. Pat. No. 3,484,237). Unfortunately, the inclusion of high loadings of such activators can and usually does result in phase separation of the various materials within such composition.
One of the alternatives to a totally organic system of the type described in the U.S. Pat. No. 3,484,237 referred to hereinabove is the fabrication of a photoreceptor wherein the light-absorbing species is distinct from the charge carrier transporting species, see, for example, U.K. Pat. No. 1,337,228, U.K. Pat. No. 1,343,671 and Can. Patent 932,199. In the systems described in the above references, charge carrier generation and charge carrier transport are performed by distinct chemical species of the imaging member (in both the (a) binder system [U.K. '671] and (b) layered systems [U.K. '228 ]). In these two latter references the electronic events of charge carrier generation and transport are performed by separate but contiguous layers. The relative order of these layers vis-a-vis one another is immaterial, however, in the preferred embodiments of the disclosed inventions, the light-absorbing, charge carrier generating layer is sandwiched between the conductive substrate and the charge transporting layer. The charge carrier transporting layer can comprise either a polymeric material or a nonpolymeric material. In the event that the nonpolymeric materials are selected, the use of such materials with a binder is generally preferred, if not required. This binder may be "electronically inert" (that is, incapable of substantial transport of at least one species of charge carrier) or can be "electronically active" (capable of substantial transport of at least one species of charge carrier). The use of such nonpolymeric materials in combination with a binder for the charge carrier transporting layer of such a device is beset with a number of problems. For example, where the binder is an electronically active polymer, such as N-vinylcarbazole, the oxidative stability of such material is rather short lived and thus, the electronic properties of the device will show a steady and progressive decline. In the event of selection of an electronically inert polymer as the binder, the resulting composition can at best be described as metastable and will also show a progressive decline in electronic properties. In this second binder system, such problems are related to the relative solubility of the nonpolymeric materials in the inert polymer and the relative stability of such solutions. Such instability is believed to be due at least in part to the tendency of nonpolymeric materials to migrate within the polymeric binder and thereby results in phase separation due to crystallization.
In light of the discussion immediately preceding, it appears that the problems inherent in such multi-layered photoconductors would preclude their use in copying systems requiring repeated cycling of the imaging member over an extended period of time since the electronic properties of the imaging member would not be capable of remaining within the machine specifications for such a device.
In addition, where such multilayered photoconductors are flexible, the continued flexing of the photoconductive composite can result in separation of the various layers from one another, thus destroying the electrical integrity and often the physical integrity of the imaging member. Accordingly, it is the object of this invention to remedy the above as well as related deficiencies in the prior art.
More specifically, it is the object of this invention to provide a method for preparation of a photoconductive composite wherein the various layers of said composite are substantially integral with one another.
It is another object of this invention to provide a method for preparation of a photoconductive composite having rapid and efficient transport of charge carriers of both polarity.
It is still yet another object of this invention to provide a method for preparation of a photoconductive composite in a manner so as to isolate the oxygen sensitive components of said composite from exposure to the environment.
Additional objects of this invention include the use of such photoconductive composites in electrophotographic devices and systems.